To what extent does the environment influence epigenetic modifications and their transmission ?

One set of examples of environmentally-programmed changes over a lifetime comes from social insects, such as bees and ants, where nutrition or pheromones can give rise to entirely different characteristics/phenotypes (soldiers, ordinary workers and queens) from the same genome. In bees, epigenetic factors that have been implicated in changes leading to queen development (when larvae are intensively fed royal jelly) or worker formation (if larvae receive bread pollen), include DNA methylation, histone acetylation (HDAC inhibitors). In the plant kingdom, the example of epigenetic changes programmed by the environment during the cold-induced flowering period (vernalization) has been discussed. Polycomb proteins have been shown to play an important role in memorizing the cold phase that defines the exact moment when a plant flowers. Recent studies suggest that winter heat spikes may interfere with this critical process. The implications of climate change for the plant and animal kingdoms have been discussed. Thanks to the many technologies available today, including -omics, imaging, as well as genetic engineering, it is possible to gain a deeper and more functional understanding of the molecular pathways involved in environmentally-induced phenotypic changes. Scientists can explore which changes in gene expression and epigenetic states are the cause or consequence of environmental signals, and what influence unprogrammed changes or environmental insults can have on an organism's development, growth and survival.

The second part of this lecture addressed several questions. What influence does the environment have on epigenetic modifications, their transmission and the transmission of phenotypes ? Can the environment induce or influence epialles ? In most cases (in mammals), induced generational changes turn out to be short-lived, i.e. intergenerational. Nutritional and metabolic changes in an individual can clearly influence its offspring and the latter's germ line. New knowledge has recently been acquired about the molecules (small RNAs) and metabolites that carry this information in mice. Epigenetic mechanisms have been implicated in the intergenerational transmission of metabolic diseases. Human and animal studies indicate that environmental exposures in early childhood can influence disease risk in adulthood. A growing body of evidence suggests that obesity and related metabolic problems can be transmitted from one generation to the next by non-genetic mechanisms. Exposure to a high-fat diet alters epigenetic information in eggs and sperm, making offspring more prone to obesity. How this happens is still unclear, but recent data suggest that metabolic pathways can disrupt chromatin states and small RNA profiles in the germline, leading to subsequent defects in offspring.

Longer-term transmission of nutritional and environmental stresses, over several generations, has been observed in nematodes. In these organisms, there is a clear transgenerational transmission following environmental stresses. A recent study was presented, in which a central role for histone modifications was proposed to explain the inheritance of reproductive defects triggered by common environmental chemical exposure.

Finally, the controversial question of whether parental psychological stress can impact offspring via epigenetic mechanisms was addressed.

The hypothesis that epigenetic marks of trauma can be passed on from one generation to the next has attracted a great deal of public interest. But the evidence, at least in humans, remains circumstantial at best. It has also been proposed that stress during pregnancy is associated with increased risks of autism spectrum disorders, schizophrenia, affective disorders and attention-deficit hyperactivity disorder in offspring, and that it is related to the specific stage of pregnancy during which the stress was experienced. However, whether and how such stress can affect the developing brain epigenetically, and whether there is an impact on the germline, remains unclear and is the subject of ongoing research.